For many applications in electronics, integrated resistors are part of a monolithic integrated semiconductor circuit. The resistors are conveniently made in the same process steps as the rest of the integrated circuit. This has, among other things, a cost saving advantage.
FIG. 1 is a simplified top view of an integrated circuit having integrated resistor 10. X and Y-coordinates are provided for convenience. Resistor 10 is formed by a current conducting well (first doping type, e.g., n-well) from X=0 to X=L diffused into substrate 20 (second, opposite doping type, e.g., p-substrate). A voltage is applied between edge 12 at X=L (e.g., positive) and edge 14 at X=0 (e.g., ground potential).
An integrated resistor is also explained by a cross section view (X and Z-coordinates) in [1] Dieter Sautter, Hans Weinerth (editors): "Lexikon Elektronik und Mikroelektronik", Zweite Auflage (second edition), VDI Verlag, Dusseldorf 1993, pages 1131-1132, figures a) and b).
However, a voltage dependent space charge region in substrate 20 limits the current path so that such resistors exhibit an often unwanted non-linearity. In other words, resistor 10 has depletion regions 16 (hatching) whose size depends on the location (e.g., along X-axis) and on the voltage V=f(X). For example, comparing resistance magnitudes at high and low applicable voltages, the resistance magnitudes R=f(X,V) differ by about 2 to 5% or even more.
Nonlinearity can sometimes be accommodated, for example, by calibration of resistors and digital correction by further stages which receive signals from the resistors. But this is limited to cases where a calibration step is possible. A further useful reference for this is: [2] Ali J. Rastegar, Janusz Bryzek: "A High-Performance CMOS Processor for Piezoresistive Sensors", Sensors, October 1997, pages 82-87.
For many applications, the non-linearity of an integrated resistor can limit the overall performance of the complete application or can prohibit the use of an integrated resistor. (a) For example, in a sensor, where the integrated resistor transforms a change of a first, non-electrical quantity (e.g., pressure, temperature, mechanical stress, acceleration) into a corresponding change of a second, electrical quantity (e.g., voltage, current signal), the resistor should transfer substantially only the first quantity changes. (b) In another example, in analog-to-digital converters (ADCs, or vice versa: DACs), resistor chains are used as voltage dividers in the analog portion to provide fractions of high (e.g., "rail-to-rail") supply voltages. Since the fractions (e.g., 20% of the supply) are feed back to the converter input, the resistor accuracy is of primary importance.
The present invention seeks to provide solutions which mitigate or avoid these and other disadvantages and limitations of the prior art.